1. Field of the Invention
This invention relates to fabrication of integrated circuits, and more particularly to an overlay mark, to a method of using the overlay mark to check a local alignment between a lower layer defined by N (≧2) exposure steps and an upper layer, and to a method that is based on use of the overlay mark to control overlay of an upper layer to be defined over a lower layer that has been defined by two exposure steps.
2. Description of the Related Art
To check the overlay accuracy between the patterns of lower and upper wafer layers that is more important as the linewidth gets smaller, a wafer is formed with many overlay marks in its non-die areas. As shown in FIG. 1, on a wafer 10, a plurality of overlay marks 100 are formed at selected positions in the scribe line regions 14 between the dies 12. A conventional overlay mark 100 is the so-called AIM (advanced imaging mark) type overlay mark as shown in FIG. 2.
Referring to FIG. 2, the overlay mark 100 includes 4 adjacent regions 102-108 arranged in a 2×2 array including a first region 102 and a second region 104 arranged diagonally and a third region 106 and a fourth region 108 arranged diagonally, and includes a portion of the lower layer defined by a lithography process for defining the lower layer and a patterned photoresist layer formed in a lithography process defining the upper layer. The portion of the lower layer includes parallel x-directional linear patterns 110 in the first region 102, parallel x-directional linear patterns 112 in the 2nd region 104, parallel y-directional linear patterns 114 in the third region 106, and parallel y-directional linear patterns 116 in the fourth region 108. The patterned photoresist layer includes parallel x-directional photoresist bars 118 in the region 102, x-directional photoresist bars 120 in the region 104, parallel y-directional photoresist bars 122 in the region 106, and parallel y-directional photoresist bars 124 in the region 108.
The linear patterns 110-116 and the photoresist bars 118-124 are arranged such that when the upper layer is fully aligned to the lower one, the central line of respective central lines of the x-directional linear patterns 110 and the x-directional linear patterns 112 coincides with that of respective central lines of the x-directional photoresist bars 118 and the x-directional photoresist bars 120, and the central line of respective central lines of the y-directional linear patterns 114 and the y-directional linear patterns 116 coincides with that of respective central lines of the y-directional photoresist bars 122 and the y-directional photoresist bars 124.
To check a local alignment at the position where an overlay mark 100 is formed, the y-coordinate “y1a” of the central line of the x-directional linear patterns 110 of the overlay mark 100, the y-coordinate “y1b” of the central line of the x-directional linear patterns 112, the x-coordinate “x1a” of the central line of the y-directional linear patterns 114, the x-coordinate “x1b” of the central line of the y-directional linear patterns 116, the y-coordinate “y2a” of the central line of the x-directional photoresist bars 118, the y-coordinate “y2b” of the central line of the x-directional photoresist bars 120, the x-coordinate “x2a” of the central line of the y-directional photoresist bars 122 and the x-coordinate “x2b” of the central line of the y-directional bars 124 are derived at first.
The method of deriving the x- and y-coordinates is exemplified by the following process of deriving x1a that is shown in FIG. 1. The y-directional linear patterns 114 are scanned by a light beam in the direction 130 to obtain a reflectivity curve 140, and respective x-coordinates of the y-directional linear patterns 114 are determined based on the reflectivity curve 140. When the linear patterns 110-116 are, for example, trenches in the lower layer, x1a is calculated as the average of the x-coordinates x1a1, x1a2, x1a3, x1a4, x1a5 and x1a6 of the six minimal points of the reflectivity curve 140.
Then, the x-directional local alignment error between the upper and lower layers at the position of the overlay mark is calculated as “(x2a+x2b)/2−(x1a+x1b)/2”, and the y-directional local alignment error is calculated as “(y2a+y2b)/2−(y1a+y1b)/2”. After the x- and y-directional local alignment errors at different wafer positions are determined with the overlay marks thereat, various types of overlay errors, such as X-translational error, Y-translational error, rotational error and magnification error, can be derived from the data of the local alignment errors, and the exposure device is adjusted to compensate the overlay errors for more accurate definition of the upper layer on the later wafers.
Moreover, when the lower layer is defined by two or three exposure steps, in the prior art, two or three above overlay marks are formed at one wafer position for the two or three exposure steps, so that x- and y-directional local alignments of the upper layer to a portion of the lower layer defined by any one of the exposure steps can be checked. Thus, a double or triple area is required to form the overlay marks.